The costs of communications spacecraft are under downward pressures due to competition among spacecraft manufacturers, and also due to competition with other forms of communications. Modularized spacecraft techniques are described in U.S. Pat. No. 5,344,104, issued Sep. 6, 1994 in the name of Homer et al.; U.S. Pat. No. 5,351,746, issued Oct. 4, 1994 in the name of Mackey et al., and U.S. Pat. No. 5,310,141, issued May 10, 1994 in the name of Homer et al. These techniques use standard modules to make spacecraft buses (payload carriers) of various sizes and capabilities, thereby reducing design costs, and particularly by reducing the need to space-qualify different structures which might be used to construct custom spacecraft using earlier techniques. Other techniques for reducing the costs of assembling buses have been implemented, such as the misalignment tolerant fasteners described in U.S. Pat. No. 5,324,146, issued Jun. 28, 1994 in the name of Parenti et al.
Payloads have been more resistant to cost reduction, because they are, almost by definition, different from each other. Each spacecraft user specifies the number of communications channels which are to be carried, their frequencies, and the power to be delivered to a specified "footprint" on the Earth's surface. The electrical power modularization required to provide the desired total radio-frequency (RF) power is described in the abovementioned patents. The antennas, however, have been more resistant. In the past, reflector/feed antennas were used on the spacecraft, with the reflector and the feed being designed to provide the desired footprint over the specified frequency range. The reflector/feed arrangement using horn feed antennas exhibits high efficiency, which is very desirable in view of the electrical power limitations common to spacecraft. However, the reflector/feed antenna is difficult to design, and multiple feed horns may be required in order to provide the appropriate footprint. Once the spacecraft is launched, the footprint cannot be changed, except by switching among the feed antennas. Also, the reflector portion of the antenna is resistant to folding, so its overall dimensions must be chosen to fit within the streamlining shroud of the launch vehicle. The size limitation, in turn, tends to limit the directive gain of the antenna, so that narrow or spot beams are difficult to obtain. Further, a reflector-type antenna is subject to physical distortion as a result of differential heating occasioned by insolation. The physical distortion, in turn, disrupts the desired footprint. Various RF-transparent insolation shields have been used to cover the radiating surface of reflector antennas, to minimize the distortion, as described, for example, in U.S. Pat. No. 5,283,592, issued Feb. 1, 1994. To the extent that the thermal (or other) antenna distortion affects the footprint, no convenient remedy is available. When operation at a plurality of different frequency ranges is necessary, as when a satellite uplink and downlink are at different bands, such as C and X band, multiple reflector antennas are required, which exacerbates the abovementioned problems. Further problems arise from the "frequency reuse" operating method, used to maximize the number of separate channels which may be used within each band, by transmitting alternate channels of each band with different polarizations, and using a polarization-sensitive reflector/feed arrangement, as described in more detail in, for example, U.S. Pat. No. 5,023,619, issued Jun. 11, 1991 in the name of Balcewicz, in that the reflector structure is much more complex than in a simple continuous reflector.
The considerations relating to reflector/feed antennas have directed attention to other types of antennas for communications spacecraft, notably antenna arrays. Antenna arrays are well known in the art, and their use in conjunction with aircraft and spacecraft is well known, although the number of such arrays in actual use in spacecraft is very small, due to a number of practical problems. Among these problems is that of the size, weight, complexity, and the attenuation or loss of the beamformer, such as that described in U.S. Pat. No. 5,274,839, issued Dec. 28, 1993 in the name of Kularajah et al., which is required to feed the RF signal to the antenna elements. Also, an array antenna must maintain a predetermined spacing between each antenna element and other elements of the array to prevent grating lobes.
Those skilled in the art know that antennas are reciprocal linear devices, in which the transducing characteristics during transmission and reception are the same. For example, the beamwidth, the gain (or more properly, the directive gain relative to an isotropic source) and the impedance at the feed points are the same in both transmitting and receiving modes. However, the terms used to describe antenna functions and characteristics were established at a time when this reciprocity was not apparent, and as a result the terms are suggestive of either transmission or reception, but generally not of both. Those skilled in the art know, therefore, that the description of an antenna may be couched in terms of either transmission or reception, or an intermixture of both, with the other mode of operation being understood therefrom. Thus, the term "feed port," for example, refers to the port to which signal energy is applied during transmission, and is also applied to that same port at which signal energy is received in a receiving mode.
Array antennas are of two general types, active and passive. The "active" antenna array includes active devices such as semiconductor devices to aid in reception or transmission, or both; a passive antenna array does not. The proper phase characteristics between the elements of the array must be provided in some way in either the active or passive arrays. An active antenna array will generally include controllable phase-shifters which can be used to adjust the phase of the RF signal being fed to one (or to a subset) of the antenna elements of the array. The need for a phase-shifting beamformer may be avoided by using a non-phase-controlled signal amplitude divider, in conjunction with control of the phase control elements associated with each element or subset of elements. An active antenna array will often have a transmit amplifier and a receive amplifier associated with each antenna or subset of antennas. These amplifiers add to the cost and complexity of the system, and are a major source of waste heat, which adds to the insolation heat, and must be taken into account. The cumulative effect of the heat absorbed by the array antenna, and that generated within the array antenna, tends to raise the temperature gradient of the array antenna, and to cause physical distortion, which in turn affects the radiation pattern and the resulting footprint. In general, antenna arrays for use in spacecraft have the disadvantages of weight, RF signal losses, and, in active embodiments, the energization power and waste heat removal requirements. The advantages of array antennas include the ability to control the beam characteristics by remote control of the phase shifters. Also, an array antenna may be folded for launch and then deployed, as described, for example, in U.S. Pat. No. 5,196,857, issued Mar. 23, 1993 in the name of Chiappetta et al.
Improved spacecraft antenna structures are desired.